1. Field of the Invention
This invention relates to strut assemblies and particularly to strut assemblies for interconnecting parts of a structure for transmitting operational loads and for introducing forcing loads as part of an active vibration reducing system.
2. Description of the Prior Art
GB-A-2160840 describes an active vibration reducing system in which a plurality of force actuators are connected at or across locations between parts of a structure which are capable of relative movement at dominant vibrating forcing frequencies. A number of sensors measure the vibration response at key locations on the structural part in which vibration is to be reduced and the resulting signals are fed to an adaptive computer/controller which provides optimal signals to the actuators to produce forces at the sensor locations in the structural part. The actuators are powered by pulsed fluid supplies, preferably hydraulic supplies.
In such a system it is essential that the interconnected parts of the structure possess different stiffness and mass characteristics so that actuator forces are reacted by the part having the greater stiffness and mass to impart movement to the other part of the structure having the lower stiffness and mass and in which vibration is to be reduced.
The aforementioned vibration reducing system is particularly suited for use in helicopters in that a fuselage structure in which it is desired to reduce vibration is generally constructed to be lightweight and flexible and is attached beneath a gearbox and rotor structure that has a high mass and is constructed to be as rigid as possible. Not surprisingly then, an exemplary embodiment of GB-A-2160840 envisages connecting the force actuators between the gearbox and the fuselage although it is clear that other locations are possible. Furthermore, whilst such a system, known as a dual point actuation system, has been used the actuators can alternatively be connected between the fuselage structure at one point and a seismic mass to generate the required forcing. Such a system is termed a single point actuation system.
The system of GB-A-2160840 has been demonstrated on the Westland 30 helicopter and that installation is described in a paper presented at the 15.sup.th European Rotorcraft Forum in September 1989 titled "An evaluation of active control of structural response as a means of reducing helicopter vibration". In the Westland 30 the gearbox and rotor are mounted on a raft connected to the fuselage by four elastomeric units which provided an ideal location for the force actuators, and four electro-hydraulic actuators were incorporated in modified elastomeric units in which the actuators operate in parallel with the elastomer spring. Such a system of force actuators is however not widely applicable since in many helicopters the gearbox is attached directly to lift frame members in the fuselage either through a plurality of gearbox mounting feet or a plurality of external angled strut assemblies.
Such a strut assembly is disclosed for use with one embodiment of a prior single input/single output vibration reducing system that is the subject of GB-A-1182339. Thus, FIG. 7 of that specification discloses a strut assembly incorporating an electro-hydraulic jack straddling a metallic oblate elastic ring which in operation is required to transmit primary lift and manoeuvring loads whilst also permitting the jack to input forcing loads to reduce vibration.
Problems with the prior strut assembly are that the oblate elastic ring represents an inefficient primary load path and it is difficult to tailor the bending loads in the ring to suit operational requirements in respect of both mechanical strength and elastic properties. The elastic ring requires a large space envelope and is bulky and heavy which may also frustrate installation in some applications.
These problems would be multiplied in attempting to utilise the strut assembly of GB-A-1182339 in the more sophisticated vibration reducing system of GB-A-2160840 because of the necessity for a plurality of jacks all inputting forcing loads simultaneously.
The strut assembly disclosed in our co-pending European Patent Application No. 92301331.2 (EP-A-0501658) overcomes these problems by locating an actuator axially within an axially elastically extensible tube provided with attachment means at both ends. However, in installations in which the available space restricts the overall length of our prior strut assembly it may not be possible to obtain a desired axial elastic extension of the tube without an unacceptable reduction in the thickness of the wall of the tube which could adversely affect its required strength in the primary load path.
An objective of this invention is therefore to provide a strut assembly which overcomes these problems.